Back in 1977, archaeologist Carl Gustafson – then at Washington State University in Pullman – excavated a male mastodon near Sequim, Washington. Buried in one of the ribs he found a bone fragment that didn't belong to the elephant-like animal, which he suggested was from the tip of a weapon used to kill the beast. Carbon dating of the remains revealed a surprise: they appeared to be around 14,000 years old – predating humans' first arrival in North America, according to the theories of the time.

Other archaeologists were unconvinced. "The dating was tenuous," says Michael Waters of Texas A&M University in College Station. Since then, however, evidence has grown that humans were in the Americas before the rise of the Clovis culture, prompting Waters and Gustafson to reanalyse the remains with the latest carbon-dating technology.

They have confirmed that both the skeleton and the bone fragment are 13,800 years old. Detailed CT scans reveal that the bone had been sharpened to a point and driven into one of the mastodon's ribs. Waters thinks the sharpened bone came from the tip of a weapon that was thrust into the animal by a hunter who was aiming for the lungs, but missed.

DNA and protein from the sharpened bone show that it came from another mastodon. To get it, the humans must have either killed one or scavenged a fresh carcass.

Munching on mastodon

Waters thinks the first American colonists came over the Bering land bridge from Asia, and may have reached Alaska as early as 20,000 years ago. From there they headed south 16,000 years ago, eventually giving rise to the more advanced Clovis culture, which used distinctive stone tools, in what is now the south-east US. A second, seafaring society may have arisen on the California coast.

"The vast majority of archaeologists now accept the pre-Clovis colonisation of the Americas," says Dennis Jenkins of the University of Oregon in Eugene.

The first colonists probably contributed to the mass extinction of large animals like mastodons, which died out as the ice age ended 12,000 years ago. "Changing weather and ecology contributed substantially to the extinction of the megafauna," says Jenkins. "However, there is no doubt that hunting by humans hastened their demise."

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This CT scan shows the weapon's point piercing the mastodon bone (Image: Center for the Study of the First Americans, Texas A&M University)